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Unless you want to rebuild that chimney, youre going to need a liner kit for the WH, especially if the chimney has 3 sides exposed. It may be difficult for that WH to keep the chimney warm enough to geta good draft.

Now the freaks are on television, the freaks are in the movies. And its no longer the sideshow, its the whole show. The colorful circus and the clowns and the elephants, for all intents and purposes, are gone, and were dealing only with the freaks. - Jonathan Winters

Simply, code says the flow area of the chimney cannot be more than 7 times the area of the drafthood outlet. If I did it right 5" round is 19.6 square inches. 12x8=96 sq inch chimney. 19.6 x 7 = 137 sq inches. Since the 96 sq inch chimney is smaller, Natual Fuel Gas Code says OK. Local codes might disagree.

Simply, code says the flow area of the chimney cannot be more than 7 times the area of the drafthood outlet. If I did it right 5" round is 19.6 square inches. 12x8=96 sq inch chimney. 19.6 x 7 = 137 sq inches. Since the 96 sq inch chimney is smaller, Natual Fuel Gas Code says OK. Local codes might disagree.

If there is a draft inducer involved, line the chimney period.

Our local inspector isn't smart enough to be able to figure that one out............LOL! Around here he needs a liner.

orphanned water heater

You need a Level II inspection first. That will determine what else in addition to the liner must be done. The liner is required to meet the class of service. It is sized per the gas codes and manufacturer's listed instructions. Once relined, it will contain the flue gases, have a stronger draft and be less prone to flue gas spillage.
HTH,
Hearthman

Simply, code says the flow area of the chimney cannot be more than 7 times the area of the drafthood outlet. If I did it right 5" round is 19.6 square inches. 12x8=96 sq inch chimney. 19.6 x 7 = 137 sq inches. Since the 96 sq inch chimney is smaller, Natual Fuel Gas Code says OK. Local codes might disagree.

If there is a draft inducer involved, line the chimney period.

Natural draft for the water heater and present furnace (vintage 1958).

B vent into chimney. 12 X 8 chimney internal area is defined as 63.6 sq in per Table G-1. Table 11-3 5" flue with 25 foot tall chimney is within natural flow limit. 1996 gas handbook says that Tables 11-11, 12 and 13 are to be ignored for water heaters. My obsolete gas code handbook is from 1996. Just wonering if anything has changed in this code area since 1996.

Anyhow, I did talk to the gas inspector (MA). He indicated that if the unit does vent properly when he tests it, then it should pass inspection.

It looks like I have to add 4 CO detectors to the house if I go direct vent furnace. Can't pass inspection without them!